|NO! DO NOT DO THIS!|
Perfumes do change with age. The most volatile notes, the top notes (so, the citrussy ones) will go first. Some brands seem to age worse than others. However, all that stuff about having to use perfume within two years of opening or it'll go bad? That is, as they say in Norfolk, a load'er old squit. I've still got the bottle of Obsession my first proper boyfriend bought me in about 1991, and it's still good. I've also got a bottle of Estee Lauder Blue Grass from the 1950s, and that's jolly nice, though as I haven't had it from new I can't tell you if it's changed massively. The one I own that has turned (and really should throw out) is a bottle of 4711 – the classic eau de cologne, it's mostly top notes, so no surprise that it went before the heavier Obsession. It smells pretty nasty now.
Perfumes go bad because they oxidise, and anyone with even a vague grasp of chemistry will realise that means oxygen is part of the process. Splash bottles age faster than spray ones, because it's easier for fresh air to get in contact with the perfume. Make sure you keep your caps on tightly. (That'll also prevent evaporation; if you regularly notice a lovely smell of perfume in your bedroom that could well be your fragrances vanishing into the atmosphere.) The other thing anyone with a vague grasp of chemistry knows is that heat speeds up a reaction, and that means two of the worst enemies for your perfumes, whatever their vintage, are heat and light.
That pretty dressing table display? Make sure it's empty bottles only, or something you won't mind losing. Sorry.
My perfumes are all, with the exception of a few bottles I acquired before I knew any better, kept in their boxes. The treasured, long-discontinued Jean Patous even live in their boxes inside a wooden box. My bedroom isn't heated – my lungs can be a bit rubbish and a heated bedroom doesn't help – but it does get hot in summer, so early every summer I transfer the whole lot downstairs to the much cooler dining room. Given the number of bottles I own, it's a migration on a par with wildebeeste crossing the Masai Mara... Serious perfumistas keep theirs in a fridge, but I don't have the space for that. At any rate, keep them dark, keep them cool.
'Dark and cool' should also be your mantra when shopping for vintage perfume. If you can pick up a bottle cheaply in a junk shop or at a car boot, it's probably fair to take a gamble, but if you're shopping for serious vintage scent, such as a decades-old bottle of something from Guerlain or Chanel, the prices will be high and you should be cautious. (Think of it as akin to buying a vintage dress from a car boot or shopping for vintage designer clothing; you'd be far fussier about condition in the latter case.) I personally would never buy a bottle that's been on display in a shop window, and would have a serious conversation with the vendor about anything that was displayed unboxed in a cabinet. While some fragrances are very dark even when new – Youth Dew springs to mind – you should avoid anything that looks darker than you'd expect. If the bottle is part-empty, ask yourself, 'Has this been used up, or do I think it has evaporated and so is likely to be oxidised?' If you're satisfied that the perfume has been stored in a reasonable manner, and it looks okay, ask to give it a sniff, and if it doesn't smell turned, see if you can try a dab on. After that, buy it if you like it. Some top notes may have gone, or it may have changed, but as long as you like it, that's the important thing!
If you like vintage and perfume, I recommend Goody's blog (food, vintage, perfume – what's not to love?). Her acquisitions make me deeply envious. And if you're looking for perfumes made now but formulated in the past, I've made a guide to scents that are still available: up to 1940, 1940-1959 and 1960-89.